ME224 Final Project: Closed-Loop Rotational Speed Control and Power Monitoring of Cooling Fan under Varying Conditions December 7, 2001 Shad Laws Karl Stensvad Jay Gainer David ChoiTable of Contents Page Summary 3 Introduction 4 Theory 5 Mechanical Setup 6 Electronic Instrumentation 7 Software 16 Data and Analysis 18 Conclusion 25 References 26 Biographical Sketches 27 Appendix 28Summary The main objective of this project is to design and build a control system for a computer fan. The user inputs a desired angular velocity in RPM into the system, and the computer alters the voltage sent to the fan in order to maintain a constant angular velocity. We then show that this closed-loop control system is functional by inputting different angular velocities, and varying the airflow resistance to the fan. Through all of this, the control system should approximately keep the RPM of the fan constant. We also want to find the power consumption of the computer fan and see if it increases as we add resistance to the airflow. From this power consumption data, we can determine the efficiency of the fan. To accomplish our goal, we built a mechanical setup that could introduce varying levels of resistance to the airflow. We also built a nice homemade encoder using a light and a photoresistor to give two pulses every revolution of the fan. Through some other circuitry, we turned that signal from the photoresistor into a rectified square wave. This then went into a tachometer, basically converting a frequency into a voltage. After some more signal conditioning, this voltage was inputted into the computer via the ADC. After multiplying by a calibrated conversion factor, we find the angular velocity of the fan. In similar ways, the current across the fan is inputted to find the power used, and the voltage outputted by the DAC were both inputted into the ADC. The data in LabVIEW was dumped out into Excel and further analyzed. In the end, our setup and circuits worked beautifully as planned. The encoder setup with the light and photoresistor gave us a very clean square wave, making the frequency easy to find. At times, the minute details of all the circuits and the shear number of wires made things difficult and confusing, but all in all, we were able to work through all of our problems. As expected the efficiency of the fan decreased as we added resistance to the fan and increased the angular velocity. The system did a superb job of maintaining the desired angular velocity as well. Though working on this project we put together all of the knowledge about various circuits and LabVIEW to create a meaningful closed-loop control system.Introduction The general goal of this experiment is to model a simple spinning fan. To do this, we have designed an electronic circuit interfaced with LabVIEW through a Data Acquisition Card. To accomplish our goal, we first built a circuit with a known resistance in series with the fan. The voltage drop across the resistor was measured to determine the current being sent through the fan. The speed (RPM) of the fan will be controlled by automatically varying the voltage to this series circuit in a closed-loop operation. More specifically, when the desired fan speed is greater than the actual fan speed, a higher voltage will be sent to the fan in order to speed it up. Conversely, when the fan speed is higher than the desired fan speed, a lower voltage is sent to the fan. In order to monitor the fan speed we built another circuit. This circuit consists of an LED and a phototransistor. In short, the phototransistor receives the LED light signal twice every rotation. This raw data was sent to LabVIEW and converted into rotations per minute using a few simple equations. We, then, wrote a program in LabVIEW that takes these data (V, I, and ? ) and exports them into an Excel file where the data can be represented visually. Next, we placed obstructions in front of the fan to block the airflow into the fan. We are looking for a relationship between the desired rpm and the power consumed by the fan. We will also try to find a relationship between fan efficiency and rpm. Since the flow of a fan is roughly linear (if the restrictions in the path of the air are constant) with respect to angular velocity with laminar flow, this can be used to estimate the efficiency of the fan.Theory After doing some background research on simple fans, we found a few relationships worth noting. First, the airflow out of the fan should be linearly related to its rotational speed. Next, the power used by the fan is related to the square of the rotational speed. So, for every increase in rotational speed, the power of the fan will increase by a factor of the square root of that increase. We also learned that, theoretically, the fan efficiency decreases as the rotational speed increases. This experiment will be conducted six different times. Each time, there will be a different amount of airflow obstruction in front of the fan’s air intake. Different sized plexiglass plates will be placed in front of the fan to act as the airflow obstructions. The plates will force the fan to work harder in order to maintain the desired rotational speed. So, as the plates increase in area, the power of the fan will increase. We also learned, and hope to prove, that as the airflow obstruction increases, the efficiency of the fan should decrease. By running this experiment with the plates of different cross sectional areas, we will try to verify the relationships that we found in our research.Mechanical Setup The fan portion of the setup was already pre-built for us – a 12 Volt 0.3 Amp fan. The components that had to be designed and built included the airflow resistance device, and the encoder setup. The fan is encased in an aluminum box made out of bent sheet metal and riveted together. There is a slot on the back where different Plexiglas plates can be inserted, cutting down the amount of airflow to the fan. There were several plates with openings of: zero, one, two, three, and nine square inches. The encoder is made up of two aluminum tubes, one with a 15 Volt light bulb, and the other one with a photoresistor. We use aluminum tubes so that the light can be reflected with less of a loss to the environment. The fan also has a hollow tube that spins with itself. At exactly two times per revolution, there is a direct line of sight between the light bulb